In a semiconductor device such as a MOSFET, or the like, the die is commonly terminated at a peripheral area by a series of spaced diffusions that form spaced PN junctions referred to as guard rings or field rings. Conventionally, these diffusions are electrically contacted to one or more field plates. Such termination regions are particularly important in power devices because of the large currents and large electric fields involved. A field plate is typically comprised of a metal, such as aluminum, or other conductor materials, and extends the electric potential forces at the depletion layer to beyond the edge of the field plate, thus reducing the depletion layer curvature and the intensity of the electric field. Diffusions may also be coupled with an equipotential ring (EQR) made of aluminum or other metals or conductive materials.
It is important to keep humidity, moisture or other contaminants from entering into the active regions of the die at the field plate or at the EQR. In some applications, the die may be used in environments that are prone to be humid, and in any event, seepage of moisture into the die may deleteriously affect the integrity of the die or its functioning within acceptable or recommended parameters.
FIG. 1 shows a termination region 31 of the die lying at a peripheral area of the active region 30 of the die. The die for example, may be a vertical conduction MOSFET, a lateral conduction MOSFET, another type of power die or some other type of semiconductor device. An electrode 20, such as a source electrode, at an upper surface of the active region 30 is also shown.
In the termination region 31 shown in FIG. 1, a number of floating field rings (FLRs) 10, sometimes referred to as guard rings, and field ring 10a are shown. Floating field rings 10 and field ring 10a are diffusions of a conductivity opposite to the conductivity of the surrounding body or layer in which they are formed. Thus, for example, floating field rings 10 and field ring 10a may be P-type diffusions formed in an N-type surrounding body.
Field ring 10a is electrically connected to a metallic field plate 22 located at or near an outer surface of the die. The field plate 22 may be made of aluminum, other conductive metal or metals, or other conductive materials. Also shown is a polysilicon element 23 in electrical contact with the metal field plate 23. A metal equally potential ring (EQR) 24 is shown electrically connected to diffusion ring 11 in the termination region 31. Also, polysilicon element 25 is electrically connected to the metal EQR 24. A passivation layer 28 is deposited on the upper surface of the die, including on the source electrode 20, the metal field plate 22 and the EQR ring 24.
In such a conventional arrangement, moisture, condensation, humidity, or other contamination may ingress into the device because the metal field plate 22 and the EQR 24 must be in direct electrical contact with the field ring 10a and the diffusion region 11.